The invention described herein relates to nuclear reactor fuel assemblies and particularly to an improved design of top nozzle used for protecting the fuel assembly against damage caused by hydraulic lift forces and differential thermal expansion of fuel assembly parts.
Conventional designs of fuel assemblies used in commercial reactors include multiple fuel rods and control rod guide tubes held in spaced relationship with each other by grids located along the fuel assembly length. This assembly of fuel rods and guide tubes is attached to a lower nozzle which rests on a bottom core plate in the reactor. The upper end of each assembly is attached to a top nozzle which is spring biased into engagement with the bottom surface of an upper core plate. Since coolant flows at high velocity against the lower end of the assembly and then upwardly therethrough, the hydraulic forces are occasionally sufficiently great to lift the assembly and the resulting hydraulic load then must be absorbed in the reactor. Also, since the fuel assembly is more than 13 feet in length, the differential thermal expansion between the assembly components and reactor core barrel in which they are located, must be accommodated in the assembly.
In prior designs, springs were used in the top nozzle for accepting the loads, as they are in the present invention, but the design was such that the springs were allowed to compress fully, or solidly, and the loads resulting from thermal expansion or hydraulic lift, then were transmitted through the top nozzle to the upper core plate. However, since the upper core plate is heavy and stationary and the spring height may vary slightly when fully compressed, the loads could distribute in a non-uniform manner among the relatively thin walls of the control rod guide tubes in the assembly. Since the distribution of the hydraulic loads could exceed the combined strength of a guide tube in a particular assembly, the guide tube could fail in compression and consequently result in great damage to the fuel rods and/or control rod guide tubes in the reactor.